Control valves play a major role in the operating efficiency of power generating systems. Such control valves must be capable of regulating the flow of fluid under extreme thermodynamic conditions. High-temperature fluid flowing through the control valve may be under high pressure and may be flowing at a high velocity. Control valves must be capable of regulating the flow of single-phase fluid where the fluid is in a gaseous state. Control valves must also be capable of regulating the flow of two-phase fluid wherein a liquid phase of the fluid at the valve inlet flashes into a mixture of liquid and steam at the valve outlet. Regardless of the form of the fluid passing through the control valve, tight shutoff of the control valve must be maintained in order to minimize valve leakage such that operating costs and maintenance intervals of the power generating system may be kept to a minimum.
Valve leakage may be caused by a significant pressure gradient across the valve seat of the control valve which can lead to cavitation. Cavitation results in the formation of vapor bubbles in the liquid flow which can cause pitting of the valve seat as well as damage downstream components. The pitting of the valve seat may result in leak paths that may increase in size over time due to erosion. Erosion may also occur as a result of fluid flowing past the valve seat of the control valve when in an open position. Excessive valve leakage may require the temporary isolation of the portion of the fluid circuit containing the leaking control valve so that the control valve may be serviced. In extreme cases, excess valve leakage may necessitate that the power generating system be taken off line in order to replace the leaking valve. Excessive leakage may also have a significant economic impact in that it may drive up the operating costs of the power generating system.
For example, in a nuclear power plant, damage to a series of control valves as a result of steam cutting along the leakage path may reduce the circumference of the valve seat of the control valves by 30%. The resulting leakage path for each one of the control valves may be about 0.19 in2 for a total leakage area of only about 1.5 in2. With a steam inlet pressure of 1100 psi at a temperature of 560° F., the steam leakage flow rate translates to over 100,000 lb/hr of lost steam. At a steam flow rate of 13,500 lb/hr/Megawatt for nuclear power plants, the loss represented by the leakage is 7.4 Megawatts of lost electrical power output. At a fuel cost of $1.00/MegaBtu, the annualized cost of the steam leakage through the control valve is over $500,000 in lost revenue. As can be seen, such a small leak may have a tremendous impact on the costs of operating the power generating system.
By monitoring the flow rate of fluid through a valve, valve leakage may be detected at an early stage such that the risk of extensive damage to the control valve and to downstream components may be minimized. Furthermore, early detection of valve leakage may help to maintain the efficiency and performance of the power generating system such that the operational costs thereof may be kept to a minimum. Thus, there exists a need for a method of monitoring valves such that valve leakage may be quantified.
The present invention addresses the above-described leakage quantification problem by providing a method for determining valve leakage based upon temperature measurements recorded upstream and downstream of the valve. More specifically, the present invention provides a method for determining the flow rate of valve leakage by correlating the heat loss of the fluid across the valve to the change in enthalpy of the fluid across the valve. The change in enthalpy is then correlated to the flow rate of valve leakage.